PROJECT SUMMARY Synapses are highly organized cellular structures that underlie neuronal communication. The formation of synapses is promoted by trans-synaptic interactions, and the adhesive complexes spanning the cleft are now understood to have instructive roles during synapse development. Our recent progress has shown that the immunoglobulin adhesion protein SynCAM 1 induces and maintains excitatory synapses, and that it contributes to shaping the synaptic cleft. The long-term goal of our research program is to define how trans- synaptic interactions organize synapse formation and maturation. The central hypothesis underlying the research proposed in this application is that synaptic adhesion molecules engage select signaling pathways to control presynaptic assembly and postsynaptic maturation, and dynamically remodel the synaptic connectivity of mature neurons in an activity-dependent manner. Three specific aims will be pursued to test this hypothesis. First, the presynaptic scaffolding and postsynaptic signaling mechanisms of SynCAM 1 will be defined. A combination of molecular and imaging approaches will be used to analyze a novel presynaptic partner, and characterize the postsynaptic regulation of small GTPases downstream of SynCAM 1. Second, the surface dynamics and activity-dependent properties of synaptic adhesion proteins will be probed. This aim employs live single particle tracking of adhesion proteins on the surface of cultured neurons. Third, it will be analyzed how synapse-organizing proteins promote synaptic connectivity during learning. We will address this question in a transgenic mouse model that allows manipulating synaptogenesis in a neuron-select manner during memory processes. These aims are significant because they determine how trans-synaptic interactions engage pre- and post-synaptic mechanisms to guide synapse development, and how these synapse-organizing complexes remodel synapses and control neuronal wiring. By establishing these fundamental aspects of synaptic biology, this research defines on molecular and in vivo levels how synaptogenic adhesion proteins control synapse formation and remodeling, enabling us to understand disorder-linked aberrations.